There have been developments in Micro-Electro-Mechanical Systems (MEMS) and microfabrication technology, which facilitates the measurement of flow rate with a miniature chip.
For one type of thermal flow sensor, it measures the effect of the moving fluid on a hot film, either through measuring the temperature change under constant heating power or monitoring the heating power adjustment for maintaining constant temperature. This type of thermal flow sensor is called a hot-wire or a hot-film sensor. For another type of thermal flow sensor, it measures the asymmetry of a temperature profile across the hot film due to the moving fluid. This type of thermal flow sensor is called a calorimetric sensor. For yet another type of thermal flow sensor, it measures the dynamic of a heat pulse passing through different locations of a hot film. This type of thermal flow sensor is called a time-of-flight sensor.
One of the components of a thermal flow sensor is the microfabricated freestanding sensing element. Heat loss from the associated heating element to the substrate during operation causes long-term sensor drift. However, to date, the problem of long-term operation sensor drift caused by heat loss has not been solved, much less for commercially available flow sensors.